A variable compression ratio mechanism of an internal combustion engine includes an operation element, an input actuator, and a reverse input torque cutoff clutch. The reverse input torque cutoff clutch includes a fixed member, a movable member, clearance, a wedge member, and a moving device. A peripheral surface of the fixed member is formed such that the clearance is formed with: a rotation prevention area that prevents the movable member from rotating in a reverse input torque acting direction. When the movable member moves in the direction to change a mechanical compression ratio, the moving device moves the wedge member from the rotation prevention area to the rotation allowable area in an opposite direction and holds the wedge member in the rotation allowable area.